In traveling-wire electroerosion processes, a continuous wire electrode is axially transported by a wire axial drive means from a supply means to a collection means. In the path of wire travel, a pair of machining guide members are commonly disposed at opposite sides of an electrically conductive workpiece to stretch or span the traveling wire electrode linearly thereacross so as to traverse the workpiece, thus positioning the wire electrode in a precise machining relationship with the workpiece. The electroerosion machine includes a power supply for passing an electrical machining current, typically or preferably in the form of a succession of electrical pulses, between the wire electrode and the workpiece across a machining gap flooded with a machining liquid, e.g. water or an aqueous solution, to electroerosively remove material from the workpiece. As the material removal proceeds, the workpiece is displaced transversely relative to the longitudinal axis of the wire electrode along a prescribed feed path under the command, preferably, of a numerical controller, so that a desired contour of cut is generated in the workpiece.
Wire electroerosion machines are very often required to machine a number of contours in a workpiece or similar workpieces in a sequence of operations. In each operation, it is necessary that machining should start with a hole preformed at a given point associated with each contour desired. Thus, prior to proceeding with any given machining operation, the wire electrode must be threaded or set through a starting hole and, subsequent to accomplishment of such machining operation, the wire electrode must be removed from the machined contour or the hole and again threaded through another starting hole associated with another contour desired to be machined in the same or another workpiece. Each rethreading operation needed after a given machining operation and before another therefore requires breaking, at a region immediately above or below the workpiece, the wire electrode extending continuously between the supply and collection sides, realigning the relative position between the said axis of the wire electrode and the worktable carrying the workpiece or workpieces, threading a broken end portion of the wire electrode through the starting hole associated with the contour to be machined and then reloading the threaded wire electrode in the wire axial drive or transpotation means downstream of the downstream guide member to establish its continuous axial travel from the supply side to the collection site.
In order to eliminate the necessity for the operator's manual intervention, an automatic wire resetting arrangement has advantageously been provided which is designed to execute the foregoing wire resetting operation automatically. In the known setup, the continuous wire electrode is broken by cutting with a blade and, prior to cutting, may be heated and thereby hardened to achieve a desired straightness of the cut end thereof. In the conventional arrangement, an anvil is disposed in contact with the wire electrode, the knife blade disposed on the opposite side of the wire electrode with respect to the anvil is moved towards the anvil to make a groove in the wire electrode urged against the anvil and then the clamping means is operated to give a tension to break the wire electrode at the site of the groove. Experience has now shown that this arrangement often results in a failure in threading the broken wire electrode through the starting hole which most desirably is as small as possible upon reestablishment of the machine setup. The failure of threading destroys the automatic resetting function and entails much and unexpected manual intervention by the operator to achieve the functional retrieval.
It has now been found that a source of conventional difficulty in the wire threading on the traveling-wire electroerosion machine lies in the configuration of grooving which has hitherto been applied in the wire electrode to be broken and reset. Thus, as shown in FIG. 3(a) in the accompanying drawing, the knife blade in the conventional arrangement has been operated to form a single groove in the wire electrode E from one lateral side thereof. When the wire electrode E grooved in this manner is stretched and broken, each end portion of the broken wire tends to bend, as shown in FIG. 3(b), due to different elongations at the grooved and ungrooved sides of the electrode respectively. Because of the thinness of the wire electrode, a considerable degree of bending thus tends to develop.
It has been found that another source of the threading difficulty is attributable to a residue of the machining liquid that locally remains adherent to the wire electrode subsequent to the previous machining operation. When the wire electrode portion prior to breaking is heated for hardening, a stress tends to develop in the wire due to irregularity in the temperature rise from one area on which the liquid remains in an greater amount to another area on which the liquid remains in a lesser amount or does not practically adhere, the stress giving rise here again to a bending of the wire on breaking.